1. Field of the Invention
The present invention relates to transmission units for transmitting drive from the engine of a motor vehicle to the half shafts of its driving wheels by means of an expansible-pulley stepless speed changer.
2. Description of the Prior Art
Vehicle drive transmission units are known which incorporate expansible-pulley stepless speed changers comprising parallel primary and secondary shafts each mounting a respective expansible pulley fast for rotation therewith. Each pulley is composed of two facing frusto-conical discs one of which is rigid with its mounting shaft and the other of which can be moved axially therealong. A trapezoidal belt passes around the two pulleys and serves to transmit drive therebetween. The radius of engagement of the belt with each pulley depends on the position of the slidable pulley disc axially of its mounting shaft. By suitable adjustment of the axial positions of the slidable pulley discs, the transmission ratio between the pulleys can be varied. Axial displacement of each slidable disc is controlled hydraulically by controlling the flow of hydraulic fluid into and out of a chamber juxtaposed the disc.
The secondary shaft is used to drive an output shaft of the transmission unit. This output shaft is connected via a bevel-gear reduction stage to a differential unit used to drive the two half shafts of the vehicle's driving wheels.
Since the expansible-pulley speed changer does not itself provide the option of drive reversal, it is generally necessary to provide transmission units incorporating such speed changers with a selectively-operable reversing unit.
In the past, drive transmission units incorporating expansible-pulley speed changers and reversing units have been rather bulky. This feature represents an increasing drawback for this type of unit since, particularly in automobile engineering, there is a trend to smaller-sized vehicles.
Accordingly, it is an object of the present invention to provide a compact drive transmission unit which incorporates both an expansible-pulley stepless speed changer and a reversing unit.